Overweight is increasingly prevalent in people living with HIV (PLWH), and is a high risk factor for metabolic disorders in this population. PLWH co-infected with hepatitis C virus (HCV) have a higher risk of metabolic disorders than their mono-infected counterparts. The putative relationship between cannabis use and body weight found in the general population has never been documented in HIV-HCV co-infected people. We tested whether cannabis use is associated with body mass index (BMI), overweight, and underweight in HCV co-infected PLWH (N = 992). Mixed-effects linear and logistic regression models were used to study the association between cannabis use and the three outcomes over time. After multivariable adjustment, cannabis use was inversely associated with BMI. Cannabis use was associated with a lower and higher risk of overweight and underweight, respectively. Cannabis use should be assessed and taken into account in the clinical management of the HIV-HCV co-infected population.